Carburization is the conventional process for case hardening of steel. In gas carburizing the steel is exposed to an atmosphere which contains components capable of transferring carbon to the surface of the metal from which it diffuses into the body of the part. A variety of atmospheres have been employed but the most commonly used one is the so-called endothermic (endo) atmosphere derived by partial combustion of natural gas in air. It is usually necessary to add a relatively small quantity of another constituent, usually natural gas, to the atmosphere to raise the carbon potential.
A thorough discussion of the Prior Art can be found in the section entitled "Furnace Atmospheres and Carbon Control" found at pages 67 through 92, and that portion of the section entitled "Case Hardening of Steel" appearing at pages 93 through 128 of Volume 2 of the Metals Handbook published in 1964 by the American Society for Metals, Metals Park, Ohio. This particular volume of the Metals Handbook is entitled "Heat Treating Cleaning and Finishing." At pages 90 through 91 of the Metals Handbook, Volume 2, there is a discussion of determination of carbon potential of a furnace atmosphere pertinent to the invention set forth below.
U.S. Pat. No. 4,049,472 also summarizes the prior art, the specification of which is herein incorporated by reference. The steel objects to be carburized are exposed at an elevated temperature, usually in the range of about 1600.degree. F. (871.degree. C.), until carbon penetration to a desired depth has been achieved. The metal can then be cooled to room temperature by various known methods such as furnace, air, and media quench to develop the desired physical properties and case hardness in the finished article. The basic endothermic atmosphere produced by the incomplete combustion of natural gas in air consists of approximately 40% N.sub.2, 40% H.sub.2, and 20% CO. The reaction by which carbon is generally believed to be deposited on the surface of the steel is represented by the following equation (1). EQU H.sub.2 +CO=C+H.sub.2 O (1)
The water produced in equation (1) immediately reacts partially with more CO according to the well-known water gas shift reaction (2). EQU H.sub.2 O+CO=CO.sub.2 +H.sub.2 ( 2)
Equations (1) and (2) may be added together to yield reaction (3). EQU 2CO=C+CO.sub.2 ( 3)
Thus, the net result of carburization by the endothermic atmosphere is the decomposition of nascent carbon on the surface of the metal and concurrent formation of an equivalent amount of CO.sub.2 or H.sub.2 O. These two substances, CO.sub.2 and H.sub.2 O, cause the reversal of reactions (1) and (3), and if allowed to accumulate would quickly bring the carburization process to a halt. The purpose of the added hydrocarbon mentioned above is to remove the H.sub.2 O and CO.sub.2 and regenerate more active reactive gases according to reactions (4a) and (4b). EQU CO.sub.2 +CH.sub.4 =2CO+H.sub.2 ( 4a) EQU H.sub.2 O+CH.sub.4 =3H.sub.2 +CO (4b)
Another method of generating a carburizing atmosphere which has been developed relatively recently, involves decomposition of methanol, either alone or in combination with nitrogen, according to equation (5). EQU CH.sub.3 OH=2H.sub.2 +CO (5)
It will be noted that the ratio of H.sub.2 to CO is 2 to 1, the same as that produced in the endothermic atmosphere by partial combustion of natural gas. By choice of appropriate quantities of nitrogen and methanol it is possible to generate a synthetic atmosphere which is essentially identical in composition to that produced by the partial combustion of natural gas. The advantages of using such a synthetic atmosphere are several fold. First, the need for an expensive and elaborate endo gas system is eliminated. The endo gas generator requires continuing maintenance and attention of an operator and furthermore it cannot be turned on and off at will. Once it is running it is necessary to keep it in operation even though the demand for the endothermic atmosphere may vary from maximum load to zero, thus the endo gas, and the natural gas required to produce it are wasted during periods of low demand. The use of nitrogen and methanol on the other hand requires only those storage facilities adequate for liquid or gaseous nitrogen and liquid methanol until they are needed. Furthermore, the nitrogen and methanol can both be injected as such directly into the furnace without the need for a separate gas generator. The methanol is immediately cracked by the high temperatures encountered in the furnace. A further advantage of the methanol-nitrogen system is that the methanol is uniform in composition while natural gas contains, in addition to methane, widely varying amounts of ethane, propane and other higher hydrocarbons which affect the stoichiometry of the partial combustion reaction and may give rise to atmospheres of substantially varying composition which in turn leads to erratic and poorly controlled behavior of the carburization process itself.
It has been shown by others, for example in U.S. Pat. No. 4,145,232, that methanol and nitrogen may be used to provide a carrier gas having essentially the same composition as endothermic gas. Others have shown, for example U.S. Pat. No. 3,201,290, that pure methanol may be used to provide a carrier gas comprised essentially of only CO and H.sub.2. A number of advantages are claimed for the latter atmosphere. First the carbon availability (the quality of carbon available for reaction per unit volume of atmosphere) is greater by a factor of 67% in the pure methanol-derived atmosphere than it is in the endothermic gas composition. This greater availability results in more uniform carburization of the workpiece since there is less likelihood of the atmosphere being depleted of carbon in regions where gas circulation is poor, for example in blind spots where several workpieces may obstruct the free flow of atmosphere in the furnace. A further advantage of the pure methanol-based atmosphere is that the kinetics of the carbon transfer are greatly enhanced. The rate at which carbon can be transferred is given by the following equation: EQU R=k.times.P.sub.CO .times.P.sub.H2
The rate of carbon transfer from a gas consisting of two-thirds H.sub.2, and one-third CO, is almost 2.8 times that of the endothermic atmosphere which contains only 40% H.sub.2 and 20% CO. Thus, it is possible to achieve more rapid carburization and lowered cycle time by the use of the pure methanol carrier gas.
However, a pure methanol-based atmosphere is inherently more expensive both in terms of monetary value and the energy required to produce it, than is an atmosphere derived in part from methanol. For example, total energy requirement to produce 100 SCF of base gas nitrogen at 1700.degree. F. (927.degree. C.) is 37,200 BTU's, while to produce the same volume of a base gas consisting of two-thirds H.sub.2 and one-third CO by decomposition of methanol 61,800 BTU's are required. These requirements include the energy necessary to heat the gas from ambient temperature to 1700.degree. F. (927.degree. C.), and in the case of nitrogen, the energy required to separate nitrogen from the air while in the case of methanol, the energy equivalent of the raw material to produce the methanol and the energy required in its synthesis and decomposition. The energy required to produce 100 SCF equivalent of synthetic endo gas from methanol and nitrogen is 51,900 BTU.
Thus it is evident that although the atmosphere derived from pure methanol is advantageous in insuring that carburization proceeds uniformly and at a rapid rate, it is more expensive and consumes more energy than does an atmosphere derived from a combination of methanol and nitrogen. The more rapid carburization achieved with the pure methanol atmosphere is desirable since it results in a shorter cycle time to achieve a given case depth, and thereby lowers the amount of energy lost through the furnace walls. However, this gain in energy conservation is to some extent offset by the higher thermal conductivity of the pure methanol-derived atmosphere as compared to the synthetic endo atmosphere because of the greater hydrogen content of the former. It is estimated that this increased hydrogen concentration results in a heat loss rate ranging from about 9% to about 14% greater for the all-methanol derived atmosphere.